1. Field of the Invention
The present invention relates to a method for producing a magnetic coating composition and a magnetic recording medium fabricated using the produced magnetic coating composition. More particularly, the present invention relates to a method for producing a magnetic coating composition using a mixing apparatus comprising an agitation means in a closed container.
2. Description of the Related Art
In general, a coating type magnetic recording medium such as a magnetic tape or a magnetic disc has a magnetic layer which is formed from a magnetic coating composition comprising a magnetic powder and small amounts of an abrasive, an antistatic agent, a lubricant, a dispersant and a hardener dispersed in a binder resins. In order to homogeneously disperse a magnetic powder consisting of comparatively fine needle-shape particles with anisotropy in a binder resin containing a solvent to prepare a highly dispersed magnetic coating, a very sophisticated preparation method of a magnetic coating is required and such method is one of the most important techniques in the development of the coating type magnetic recording medium. A degree of dispersion of the magnetic powder has a great influence on not only an electromagnetic conversion property among performances of the medium but also durability of the medium. Therefore, various dispersing methods have been studied and proposed by manufacturers of the magnetic recording media.
Recently, to satisfy requirements for high output and low noise, a coating type magnetic metal recording medium comprising, as a magnetic powder, pure iron magnetic power having a smaller particle size and larger ferromagnetism is predominantly used. To this end, for the purpose of pretreatment, the ferromagnetic iron powder is often mixed with a small amount of the binder resin using a mixing apparatus which generates larger shear force such as a kneader. The magnetic powder particles are strongly agglomerated through their own magnetic attraction force, intermolecular force, and also hydrogen bonds of hydroxyl groups or water present on the particle surfaces. As the magnetic particle size decreases and magnetism increases, the agglomerating force of the powder increases. To disperse the magnetic powder by overwhelming such force, a force larger than the agglomerating force is required. Kneading intends to throughly wet the magnetic powder with a solvent and coat surfaces of the magnetic powder particles with the binder resin uniformly. Since large shear force is applied to the magnetic powder if the magnetic powder and the binder resin are mixed in a highly viscous state, kneading is the most effective method for highly dispersing the fine ferromagnetic powder such as the pure iron powder.
As methods for producing the magnetic coating composition by kneading, methods using a batchwise kneading machine are disclosed in Japanese Patent Kokai Publication Nos. 3030/1971, 104405/1973, 14537/1974, 9102/1978, 76012/1978 and 25406/1980, and methods using a continuous kneading machine are disclosed in Japanese Patent Kokai Publication Nos. 79274/1989, 79275/1989 and 106338/1989, FUNTAI-KOGAKUSHI (Powder Technology Journal), Vol. 25, 344 (1988) and KAGAKUKOGAKU RONBUNSHU (Chemical Engineering Papers), Vol. 15, 747 (1989).
However, the conventional kneading methods of the magnetic powder forcedly pulverize and disperse the magnetic powder agglomerates with a large shear force only by relying on a strong power of the kneading machine, but neither interfacial phenomenon such as wetting of the magnetic powder particle surfaces with the solvent nor adsorption of the binder resin on the magnetic powder particle surfaces is taken into consideration. To generate strong shear force, it is necessary to reduce an amount of the solvent in the composition as much as possible and knead the composition at high viscosity. However, kneading conditions may vary according to exterior factors such as temperature and humidity or material factors, in particular, a water content, surface chemical properties and a specific surface area of the magnetic powder, so that, in some cases, the kneading is possible, while in other cases, kneading is not possible. Therefore, the composition may not be stably kneaded, or desired dispersibility of the composition may not be achieved by simple kneading under force only.
To highly disperse the magnetic powder, the binder resin should uniformly cover the magnetic powder particles. To this end, air on the surfaces of the magnetic powder particles is expelled and the surface should be well wetted with the solvent. However, the binder resin is not sufficiently adsorbed by forced kneading of the magnetic powder and the binder resin. To achieve kneading, an excess amount of the binder resin is required. But, the excess amount of the binder resin migrates onto the surface of the magnetic layer to form a thin film during calendering. Such thin resin layer causes a loss in spacing between the recording medium and a magnetic head and, in an extreme case, causes deterioration of electromagnetic conversion characteristics of up to several dB. If the composition is kneaded in the presence of too small an amount of the solvent with larger force, the magnetic powder is more strongly agglomerated, and in the worst case, the particles of the magnetic powder are broken, resulting in deterioration of the magnetic properties of the medium.
When a batchwise kneading machine having an agitation blade with a larger occupying volume in a vessel such as a kneader or an internal mixer, kneading under high shear force generates a large amount of heat since the apparatus has a small radiating surface area. Therefore, the binder resin may be decomposed or the solvent may be evaporated, which are causes for nonuniform kneading. In particular, when a cellulose base binder resin is used or the pure iron powder is used as the magnetic powder, the composition may be ignited by heat developed. Such is unfavorable in view of safety.
When the continuous kneading machine is used, powdery materials such as the magnetic powder and the antistatic agent and liquid materials such as the binder resin solution and the solvent are separately supplied. However, it is very difficult to supply them with a constant composition, since flowability of the magnetic powder delicately varies with the water content of the powder particle surfaces, and the viscosity of the binder resin solution varies with ambient temperature. Dispersion of the composition leads to dispersion of the performances of the finally fabricated magnetic recording medium, so that quality control is difficult. Since the kneading machine is not a closed one, when a magnetic material which spontaneously ignites in air such as the pure iron powder is used as the magnetic powder, a tool for purging a feeder or inlet for supplying the powder with nitrogen gas should be provided.
The kneaded composition can be diluted to some extent in the continuous kneading machine by pouring the solvent to the composition which is moving in the kneading vessel at a constant rate from a pouring opening provided in the kneading vessel. In comparison with the batchwise kneading machine in which the composition is diluted with the solvent by gradual addition of the solvent over a long time in the closed container under continuous shear, since the composition is diluted in the kneading vessel having a specific length in the continuous kneading machine, a clearance between paddles is small so that the dilution is carried out in a very short time period and sufficient dilution is not possible even with the high shear force. According to studies by the present inventor, the composition prepared by kneading often has a viscosity of several million to several ten million poise. It is very difficult to dilute such hard composition with the continuous kneading machine, and small masses of the composition tend to remain in the diluted material. Then, it is very difficult to prepare a uniformly diluted material. When the composition is kneaded with the continuous kneading machine, a separate dilution apparatus is provided after the continuous kneading. Therefore, the continuous kneading machine is not attractive in view of its workability, productivity or apparatus costs.